The present invention relates generally to catalytic bodies and more specifically to catalytic bodies for use as cathodes in an alkaline fuel cell. The catalytic body of the invention is based on a disordered non-equilibrium material designed to have a high density of catalytically active sites, resistance to poisoning and long operating life.
A fuel cell is an electrochemical device in which the chemical energy of a conventional fuel is converted directly and efficiently into low voltage electrical energy. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and remote power supply applications.
Fuel cells, like conventional batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells also offer a number of important advantages over engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle. As the world's oil supplies become depleted, hydrogen supplies remain quite abundant and offer a viable alternate source of energy. Hydrogen can be produced from coal or natural gas or can be produced without the use of fossil fuels, such as by the electrolysis of water using nuclear or solar energy.
The major components of a typical fuel cell are the anode for hydrogen oxidation and the cathode for oxygen reduction, both being positioned in a cell containing an electrolyte such as an alkaline electrolytic solution. Typically, the reactants such as hydrogen and oxygen, are respectively fed through a porous anode and cathode and brought into surface contact with the electrolytic solution. The particular materials utilized for the cathode and anode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the anode is between the hydrogen fuel and hydroxyl ions (OH.sup.-) present in the electrolyte which react to form water and release electrons: H.sub.2 +2OH.sup.- .fwdarw.2H.sub.2 O+2e.sup.-. At the cathode, the oxygen, water, and electrons react in the presence of the cathode catalyst to reduce the oxygen and form hydroxyl ions (OH.sup.-): (O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.-). The flow of electrons is utilized to provide electrical energy for a load externally connected to the anode and cathode.
Despite the above listed potential advantages, fuel cells have not been widely utilized. Contributing to the fuel cell's lack of widescale commercial acceptance has been the relatively high cost of operating the fuel cells. The most important factor contributing to the relatively high cost of producing energy from a fuel cell are the catalytic inefficiencies of the prior art catalytic materials used for the electrodes and/or the high costs of many of these materials. The catalytic inefficiencies of the materials add to the operating costs of the fuel cell since a lower electrical energy output for a given amount of fuel results. The use of expensive catalytic materials, such as noble metal catalysts, result in cells which are too expensive for widespread application.
The only alkaline fuel cells presently utilized are based upon noble metal catalysts and because of potential poisoning utilize ultrahigh purity fuels and electrolytes. These very expensive cells are only utilized for space applications where cost is not a factor. Virtually no commercial applications presently utilize alkaline fuel cells.
For example, one prior art fuel cell cathode catalyst is platinum. Platinum, despite its good catalytic properties, is not very suitable for widescale commercial use as a catalyst for fuel cell cathodes, because of its very high cost. Noble metal catalysts like platinum, also cannot withstand contamination by impurities normally contained in the hydrogen fuel and the electrolyte of the fuel cell. These impurities can include carbon monoxide which may be present in hydrogen fuel or contaminants contained in the electrolyte such as the impurities normally contained in untreated water including calcium, magnesium, iron, and copper.
The above contaminants can cause what is commonly referred to as a "poisoning" effect. Poisoning is where the catalytically active sites of the material become inactivated by poisonous species invariably contained in the fuel cell. Once the catalytically active sites are inactivated, they are no longer available for acting as catalysts for efficient oxygen reduction reaction at the cathode. The catalytic efficiency of the cathode therefore is reduced since the overall number of available catalytically active sites is significantly lowered by poisoning. The decrease in catalytic activity results in increased overvoltage at the cathode and hence the cell is much less efficient adding significantly to the operating costs. Overvoltage is the voltage required to overcome the resistance to the passage of current at the surface of the cathode (charge transfer resistance). The overvoltage represents an undersirable energy loss which adds to the operating costs of the fuel cell.
The reduction of the overvoltage at the cathode to lower operating cost of fuel cells has been the subject of much attention in the prior art. More specifically, the attention has been directed at the reduction of overvoltage caused by the charge transfer resistance at the surface of the cathode due to catalytic inefficiencies of the particular cathode materials utilized.
One prior art attempt to improve on the noble metal based catalysts was to use a spinel NiCo.sub.2 O.sub.4 material. The spinel material can be prepared as a powder by freeze drying and by co-precipitation from a solution of mixed salts. Application of the catalytic material to the electrode substrate can be accomplished by using a binder mixed with the catalysts or by dipping the electrode substrate into a solution of mixed nitrate salts which is then dried and heated to decompose the nitrates and cured.
The shortcomings of spinel catalysts, as well as other prior cathode catalysts proposed in the prior art, is that these catalysts are generally based upon a crystalline structure. In a crystalline structure the catalytically active sites which provide the catalytic effect of such materials result primarily from accidently occurring, surface irregularities which interrupt the periodicity of the crystalline lattice. A few examples of such surface irregularities are dislocation sites, crystal steps, surface impurities and foreign adsorbates. A major problem with a crystalline structure is that the number of such irregularities forming the catalytically active sites are relatively few and occur only on the surface of the crystalline lattice. This results in the catalytic material having a density of catalytically active sites which is relatively low. Thus, the catalytic efficiency of the material is substantially less than that which would be possible if a greater number of catalytically active sites were available for the oxygen reduction reaction. Such catalytic inefficiencies result in a reduction in the fuel cell efficiency.
In summary, high catalytic efficiency from a relatively low cost material and resistance to poisoning in a fuel cell environment remain as desired results which must be attained before widescale commercial utilization of fuel cells is possible. Prior art fuel cell cathode catalysts, which have been generally predicated on either expensive noble metal catalysts or crystalline structures with a relatively low density of catalytically active sites, have not been able to meet the above requirements.